“ 烂苹果 ”

If you have one spoiled （烂的） apple in a box with a lot of good apples, the bad one will soon damage the others. Because a bad person can have a negative influence on those around him or her, he or she is called a bad / rotten apple.
　　如果你把一个烂苹果和一堆好苹果都放在盒子里，那么这个坏苹果会很快也让其他的受到伤害，正如一个坏人会带坏他身边的一群人一样，那么他/她就被称为害群之马。

　　"Every school has a couple of bad apples who damage its　reputation （声誉）," Danny said. "Joseph is the rotten apple in our school."
　　“每个学校都有一些害群之马在损害学校的声誉，”丹尼说，“约瑟夫就是我们学校的害群之马。”